In general, the present invention relates to a composition of an aluminum alloy used in the fabrication of semiconductor processing apparatus. In particular, the invention relates to a structure comprising a clean aluminum alloy which is particularly advantageous when used to fabricate semiconductor processing chamber components.
Semiconductor processing involves a number of different chemical and physical processes whereby minute integrated circuits are created on a substrate. Layers of materials which make up the integrated circuit are created by chemical vapor deposition, physical vapor deposition, and epitaxial growth, for example. Some of the layers of material are patterned using photoresist masks and wet and dry etching techniques. Patterns are created within layers by the implantation of dopants at a particular locations. The substrate upon which the integrated circuit is created may be silicon, gallium arsenide, glass, or any other appropriate material.
Many of the semiconductor processes used to produce integrated circuits employ halogen or halogen-containing gases or plasmas. Some processes use halogen-containing liquids. In addition, since the processes used to create the integrated circuits leave contaminant deposits on the surfaces of the processing apparatus, such deposits are commonly removed using plasma cleaning techniques which employ at least one halogen-containing gas. The cleaning procedure may include a wet wipe with deionized water, followed by a wipe with ispropyl alcohol.
Aluminum has been widely used as a construction material for semiconductor fabrication equipment, at times because of its conductive properties, and generally because of its ease in fabrication and its availability at a reasonable price. However, aluminum is susceptible to reaction with halogens such as chlorine, fluorine, and bromine to produce for example, AlCl3 (or Al2Cl6); or AlF3; or AlBr3 (or Al2Br6). The aluminum-fluorine compounds can flake off the surfaces of process apparatus parts, causing an eroding away of the parts themselves, and serving as a source of particulate contamination of the process chamber (and parts produced in the chamber). Most of the compounds containing aluminum and chlorine, and many of the compounds containing aluminum and bromine, are gaseous under semiconductor processing conditions and leave the aluminum structure, creating voids which render the structure unstable and produce a surface having questionable integrity.
A preferred means of protecting the aluminum surfaces within process apparatus has been an anodized coating. Anodizing is typically an electrolytic oxidation process that produces an integral coating of aluminum oxide on the aluminum surface. Despite the use of anodized protective layers, the lifetime of anodized aluminum parts in semiconductor processing apparatus, such as susceptors in CVD reactor chambers and gas distribution plates for etch process chambers, has been limited due to the gradual degradation of the protective anodized film. Failure of the protective anodized film leads to excessive particulate generation within the reactor chamber, requiring maintenance downtime for replacing the failed aluminum parts and for cleaning particulates from the rest of the chamber.
Miyashita et al., in U.S. Pat. No. 5,039,388, issued Aug. 13, 1991, describe a plasma forming electrode used in pairs in a semiconductor processing chamber. The electrode is formed from a high purity aluminum or an aluminum alloy having a chromic acid anodic film on the electrode surface. The chromic acid anodized surface is said to greatly improve durability when used in a plasma treatment process in the presence of fluorine-containing gas. The electrode is described as formed from a high purity aluminum such as JIS 1050, 1100, 3003, 5052, 5053, and 6061, or similar alloys such as Agxe2x80x94Mg alloys containing 2 to 6% by weight magnesium.
U.S. Pat. No. 5,756,222, to Bercaw et al., issued May 26, 1998, and entitled xe2x80x9cCorrosion-Resistant Aluminum Article For Semiconductor Processing Equipmentxe2x80x9d, describes an article of manufacture useful in semiconductor processing which includes a body formed from a high purity aluminum-magnesium alloy having a magnesium content of about 0.1% to about 1.5% by weight, either throughout the entire article or at least in the surface region which is to be rendered corrosion-resistant, and an impurity atom content of less than 0.2% by weight. Impurity atoms particularly named include silicon, iron, copper, chromium, and zinc. The high purity aluminum-magnesium alloy may be overlaid by a cohesive film which is permeable to fluorine, but substantially impermeable to oxygen. Examples of such a film include aluminum oxide or aluminum nitride. The subject matter disclosed in this patent is hereby incorporated by reference in its entirety.
U.S. Pat. No. 5,811,195, to Bercaw et al., issued Sep. 22, 1998, and entitled xe2x80x9cCorrosion-Resistant Aluminum Article For Semiconductor Equipmentxe2x80x9d, further discloses that the magnesium content of the aluminum article may be in the range of about 0.1% to about 6.0% by weight of the aluminum article. However, for operational temperatures of the article which are greater than about 250xc2x0 C., the magnesium content of the aluminum article should range between about 0.1% by weight and about 1.5% by weight of the article. In addition, an article is described in which the impurities other than magnesium may be as high as about 2.0% by weight in particular instances. One example is when there is a film overlying the exterior region of the article body, where the film comprises aluminum oxide or aluminum. Another example is where there is a magnesium halide layer having a thickness of at least about 0.0025 xcexcm over the exterior surface of the aluminum article. The subject matter disclosed in this patent is hereby incorporated by reference in its entirety.
For an aluminum alloy to be useful in the fabrication of semiconductor processing apparatus, it must not only exhibit low level of impurity atoms, but it must also have desirable mechanical properties. The mechanical properties must enable machining to provide an article having the desired dimensions. For example, if the alloy is too soft, it is difficult to drill a hole, as material tends to stick during the drilling rather than to be removed by the drill. Controlling the dimensions of the machined article is more difficult. There is penalty in machining cost. The mechanical properties of the article also affect the ability of the article to perform under vacuum. For example, a process chamber must exhibit sufficient structural rigidity and resistance to deformation that it can be properly sealed against high vacuum. Finally, when the article is treated, to reduce stress, for example, the treatment must ensure that there is uniform transfer of loads and stresses.
The xe2x80x9cMetals Handbookxe2x80x9d, Ninth Edition, Volume 2, copyright 1979, by the American Society for Metals, describes the heat treatment of aluminum alloys, beginning at page 28. In particular, for both heat-treatable and non-heat-treatable aluminum alloys, annealing to remove the stress created during cold work is accomplished by heating within a temperature range from about 300xc2x0 C. (for batch treatment) to about 450xc2x0 C. (for continuous treatment).
In general, the term xe2x80x9cheat treatmentxe2x80x9d applied to aluminum alloys is said to be restricted to the specific operations employed to increase strength and hardness of precipitation-hardenable wrought and cast alloys. These are referred to as xe2x80x9cheat-treatablexe2x80x9d alloys, to distinguish them from alloys in which no significant strengthening can be achieved by heating and cooling. The latter are generally said to be referred to as xe2x80x9cnon-heat-treatablexe2x80x9d alloys, which, in wrought form, depend primarily on cold work to increase strength. At page 29 of the xe2x80x9cMetals Handbookxe2x80x9d, Table 1 provides typical full annealing treatments for some common wrought aluminum alloys. The 5xxx series of aluminum alloys are of interest for use in fabricating semiconductor processing apparatus because some of the alloys offer impurity concentrations within acceptably moderate ranges, while providing sufficient magnesium content to perform in the manner described in the Bercaw et al. patents. These alloys are not considered to be heat treatable.
Standard thermal stress relief of non-heat-treatable aluminum alloys such as the 5xxx series assumes peak temperatures approaching 345xc2x0 C. and generic ramp rates and dwell times, without regard to the alloy or the final use of individual articles fabricated from the alloy. Aluminum alloys begin to exhibit grain growth at temperatures approaching 345xc2x0 C., and enhanced precipitation of non-aluminum metals at the grain boundaries, which may lead to cracking along the grain boundaries during machining. The above factors also have an impact on the mechanical properties of the alloy by affecting the uniformity of the alloy composition within the article. When attempting to achieve the mechanical properties required for the aluminum alloy body of the article, it is necessary to ensure that the treatment of the alloy to improve mechanical properties will not cause other problems in the finished article.
When the article fabricated from an aluminum alloy is to be used in a corrosive atmosphere, it is frequently necessary to provide protective coatings such as an anodized layer over the aluminum surface. This is particularly true for applications of aluminum in semiconductor processing where corrosive chlorine or fluorine-containing etchant gases and plasmas generated from these gases are employed. A stable anodized layer over the surface of the specialty aluminum alloy helps maintain a halide protective component at or near the surface of the aluminum alloy. The anodized layer also helps prevent abrasion of the aluminum surface as well as any other protective layer present on the aluminum surface. The combination of the anodized layer and a halide protective component overlying the specialty aluminum alloy provides an article capable of long-term functionality in the corrosive environment. Not only is there significant expense in equipment maintenance and apparatus replacement costs due to degradation of the protective anodized film, but if a susceptor, for example, develops significant surface defects, these defects can translate through to affect a silicon wafer atop the susceptor, creating contamination on the wafer which leads to device current leakage or even short. This can affect the reliability of the device. Unreliable products can cause significant damage to a manufacturer""s reputation.
Aluminum alloy 6061 is a standard aluminum alloy typically used to fabricate semiconductor processing equipment. The 6061 alloy generally has the following impurity content by weight %: a magnesium concentration ranging from about 0.8% to about 1.2%, a silicon concentration ranging from 0.4% to about 0.8%, an iron concentration up to 0.7%, a copper concentration ranging from about 0.15% to about 0.4%, a manganese concentration of about 0.15%, a zinc concentration of about 0.25%, a chromium concentration ranging from about 0.04% to about 0.35%, with other single impurities not exceeding about 0.05% each, and other total impurities not exceeding about 0.15%.
Some of these elements can be harmful to a semiconductor device fabricated in a process chamber including the 6061 alloy. For example, a copper layer used as a metal interconnect in semiconductor circuits is desirable due to its low resistivity. However, the presence of copper impurities in semiconductor process chamber components is undesirable, because copper contamination from process chamber components onto a substrate processed in the chamber will affect the performance of integrated circuit devices which are present on the substrate.
There are some specialty aluminum alloys which have been developed for use in semiconductor processing equipment, but such alloys are particularly expensive to manufacture.
It would be desirable to have an aluminum alloy which is cost-effective to manufacture, has the desired mechanical and chemical properties, and which works well in combination with an anodized coating to provide a long performance lifetime for the processing apparatus.
We have discovered that the formation of particulate inclusions at the surface and the interior of an aluminum alloy article interferes with the performance of the article when a surface of the article is protected by an anodized coating. We have also discovered that the formation of such particulate inclusions can be controlled to a large extent by controlling the concentration of particular impurities present in the alloy itself.
In particular, we have determined that an aluminum alloy having the following impurities present by weight %, while being relatively inexpensive to fabricate, will perform well when used in the fabrication of semiconductor processing apparatus: A silicon at a concentration ranging between about 0.54% and about 0.74%, copper at a concentration ranging between about 0.15% and about 0.30%, iron at a concentration ranging between about 0.05% and about 0.20%, manganese at a concentration up to about 0.14%, zinc is at a concentration up to about 0.15%, chromium at a concentration ranging about 0.16% and about 0.28%, titanium at a concentration up to about 0.06%, and magnesium at a concentration ranging between about 0.9% and about 1.1%. The total of other impurities present in the aluminum alloy are not to exceed 0.15%, with individual other impurities limited to a maximum of 0.05% each.
In addition to the chemical compositional requirements, the aluminum alloy is required to meet particular specifications with respect to particulates formed from the impurities present in the alloy. Of the particulate agglomerations of impurity compounds, at least 85% of all particles must be less than 5 xcexcm in size. Less than 15% of the particles may range from 5 xcexcm to 20 xcexcm in size. No more than 1% of the particles may be larger than 20 xcexcm in size, with no particles larger than 40 xcexcm.
An aluminum article after extrusion into a desired shape, or an aluminum article after machining into a desired shape from a block or billet, is typically stress relieved at a temperature about 415xc2x0 C. or less. The stress relief provides a more stable surface for application of the anodized protective film and may provide improved mechanical performance properties.